Each year Americans lose track of personal property worth, in the aggregate, hundreds of millions of dollars. We fail, for example, to cash checks for stock dividends or salary payments and neglect to claim security deposits or insurance benefits. Laws in all 50 states require businesses that hold certain kinds of unclaimed property to transfer such property to the state after periods of inactivity that vary from less than one to seven years. State governments do not themselves take title to this unclaimed property, but hold the property in perpetuity as custodian for the owner or the owner's heirs and devisees. As a result of the unclaimed property laws, states have acquired many billions of dollars of unclaimed property. This article discusses how the practical effects of these laws force us to reconsider some of the theoretical underpinnings of a variety of tax doctrines, including constructive receipt, the definition of debt, and the tax benefit rule. In particular, it exposes how our substantive tax laws assume that taxpayers act intentionally, as rational actors knowledgeable about and aware of their actions. The case of unclaimed property reminds us that our tax laws apply to human behavior and that our tax base needs to allow for human frailty. It helps us to highlight those tax doctrines, such as surrogate taxation or the rules applicable to fiduciaries, that allow for human carelessness. The study further suggests that the contours of many tax doctrines be limited to voluntary, or at least conscious, actions. Of special significance would be rethinking our notion of debt to require some conscious intention on the part of the taxpayer to make a loan.